1. Field of the Invention
The present invention relates to a system for receiving DTMF signals from telephone networks.
2. Description of the Related Art
It is known in the art that dual tone multi frequency (DTMF) signals are used as destination selecting signals. The typical telephone set incorporates a device for transmitting the DTMF signals. The telephone exchange connects a telephone line to another telephone line, in accordance with DTMF signals which have been transmitted from a telephone set to the telephone line.
For example, a DTMF signal is a combination of one frequency from the group of frequencies consisting of 1209 Hz, 1336 Hz, 1477 Hz and 1633 Hz with one frequency from another group of frequencies consisting of 697 Hz, 770 Hz, 852 Hz and 941 Hz. Such combinations represent numerals "0" through "9" etc.
In recent years, the DTMF signals have been frequently used as signals for controlling various instruments. For example, if a preselected password consisting of DTMF signals is transmitted from an outdoor telephone set to an indoor telephone set, any instrument connected with the indoor telephone set (e.g. air-conditioner or video tape recorder) can be remotely controlled by the DTMF signals. The DTMF signal receiving system is incorporated, for example, into an automatic answering telephone set or the like.
FIG. 1 illustrates the operation of a conventional DTMF signal receiving system. When a DTMF signal 100 is detected by a detector (not shown), the detector outputs a detection signal 102. The occurrence of the detection signal 102 causes a control circuit (not shown) to output a data request signal 103. On the other hand, the inputted DTMF signal 100 is converted by a decoding circuit (not shown) into 4-bit decoded data which is in turn supplied to a shift register (not shown). The data request signal 103 consists of four pulses and is inputted into the shift register. Each time a pulse 103a is inputted into the shift register, the latter serially outputs one bit to a decoding circuit 104. Thus, only one data request signal 103 is outputted from the control circuit when one DTMF signal is received by the DTMF signal receiving system.
However, if an undesirable DTMF signal 100 having a discontinuous point 106 is received by the DTMF signal receiving system due to the depression of a wrong button or some other spurious operation as shown in FIG. 2, properly decoded data cannot be produced.
As shown in FIG. 2, the undesirable DTMF signal 100 includes a forward signal section 100a representing "#n" and a rearward. signal section 100b representing "#n+1", these signal sections being separated by the discontinuous point 106.
It is not unusual that when an operator intends to depress a particularly button, he falsely touches another button adjacent to the particular button. This will produce an undesirable DTMF signal 100 having such a discontinuous point 106. When this undesirable DTMF signal is received by the conventional DTMF signal receiving system, the latter finally obtains only decoded data "#n", but not desired decoded data "#n+1" which is inherently to be transmitted to the DTMF signal receiving system.
If such a DTMF signal is used to control an instrument remotely, this will lead to the instrument malfunctioning. This is also true of a DTMF signal containing noise.
It is therefore an object of the present invention to provide a DTMF signal receiving system which can obtain properly decoded data from an undesirable DTMF signal having the discontinuous point and/or noise.